1. Field of the Invention
The present invention relates to the technique of reproducing digital data only during a term that allows reproduction, particularly to the technique of detecting fraudulent modification of the current time in a reproduction apparatus including a circuit that detects the current time to strictly monitor reproduction of digital data.
2. Description of the Background Art
In accordance with the development of recent digital technology, image data and music data recorded on a recording medium in a digital form can be reproduced at the user side using a reproduction apparatus. In the case where the recording medium is available on a rental basis, the user can reproduce the video and music using a reproduction apparatus if within a preset reproduction term. Here, determination of whether reproduction is allowed or not is made based on the detected time at the user's reproduction apparatus and the reproduction time limit information recorded in the recording medium. If the current time detected by the user's reproduction apparatus is improperly modified, the content can be reproduced even if the reproduction time limit has expired. Monitoring the time detect by the user's reproduction apparatus is a critical factor.
Japanese Patent Laying-Open No. 8-235402 discloses a time recorder detecting any fraudulent modification of time. The time recorder disclosed in this publication monitors time based on the time of an internal clock. The time recorder includes a first detection circuit detecting time at high accuracy, a second detection circuit detecting time at lower accuracy than the first detection circuit, a circuit storing in-and-out time data representing the arrival and departure time based on the time detected by the second detection circuit, a calculation circuit calculating the time difference between the first detection circuit and the second detection circuit every time the in-and-out time data is input, a determination circuit determining whether the absolute value of the calculated time difference exceeds a reference time, and a circuit recording the modification history when the time difference exceeds the reference time.
The first detection circuit has the time set only by an administrator. The second detection circuit is adjusted to the time detected by the first detection circuit, when there is error in time, by an individual other than the administrator. The time recorder includes two detection circuits to detect the time, and determines whether the time is modified or not based on the calculation of the time difference obtained according to the time detected by the first detection circuit of higher detection accuracy. Accordingly, the history of the time being modified improperly is recorded when the time difference calculated by the calculation circuit exceeds a reference time.
It is to be noted that this time recorder requires a first detection circuit that detects time at high accuracy, which is expensive. Although the time can be monitored strictly in a reproduction circuit reproducing digital data, the usage of such a first detection circuit will increase the cost of the reproduction apparatus.